Fuel Economy Ratings Revised by the U.S. EPA

But don't worry. Nothing has changed with your car, fuel or the workings of new vehicles. It's just the federal government trying to correct a faulty mileage procedure that it used for years to calculate miles-per-gallon figures posted on new-vehicle window stickers and on a government Web site.

The new procedure and resulting mpg numbers become official with the 2008 model year, when they will be prominently displayed on newly redesigned window stickers.

But consumers can see what's coming already because the U.S. government has released its estimated new numbers for 2007 and earlier model years at www.fueleconomy.gov.

Indeed, consumers can even compare older vehicles' mileage ratings going back to the 1985 model year with what the government figures are under the new regimen.

Hybrid Cars Affected Most
Hardest hit in the government change is the Toyota Prius, the best-selling hybrid in the world. The 2007 Prius has a 60-miles-per-gallon rating for city driving. The new EPA calculation drops that by 12 mpg, or 20 percent, to 48 mpg. Highway mileage falls by 6 mpg, or 11.8 percent, to 45 mpg.

Note, though, that because the fuel economy ratings for virtually all vehicles are being dropped and no other vehicle had anywhere near the fuel economy rating of the Prius. Toyota's midsize hybrid sedan maintains its spot as the most fuel-efficient vehicle in America.

Remaining the second-best vehicle for fuel efficiency is another hybrid, the Honda Civic Hybrid, whose fuel economy rating also is going down.

In the government estimates, city mileage for the 2007 Civic Hybrid drops 18.4 percent, from 49 mpg to 40 mpg, while highway mileage declines 6 mpg to 45 mpg.

Hybrid SUV Ratings Changed Less
The new EPA calculations don't have as major an impact on hybrid sport-utility vehicles, which are heavier and have lower fuel economy ratings to begin with.

The 2007 Ford Escape Hybrid with two-wheel drive remains the best SUV in America in fuel economy. But under the new calculations, its fuel economy ratings drop by 13.9 percent and 6.4 percent, respectively, to 31 mpg in the city and 29 mpg on the highway.

Following the Escape Hybrid are the 2007 Toyota Highlander Hybrid with two-wheel drive and the 2007 Saturn Vue Green Line.

The Highlander Hybrid's city mpg drops 12.5 percent to 28 mpg and highway mileage declines 7.4 percent to 25 mpg.

The Vue's original 27/32 mpg rating drops to 23/29 mpg.

Be aware that Saturn stopped producing the 2007 Vue Green Line in the first quarter of 2007 in preparation for a 2008 Vue hybrid with new hybrid system and improved fuel economy. The EPA has not yet tested the new Vue hybrid.

Big cars, SUVs Fare Better
Non-hybrid vehicles—especially those that are large, heavy and already have lower fuel economy ratings—aren't seeing large drops in mileage ratings.

For example, the city mileage for a 2007 Cadillac Escalade with all-wheel drive is declining just 1 mpg, or 7.7 percent, to 12 mpg under the new EPA calculations. Highway mileage for this Escalade model doesn't change at all, staying at 19 mpg under the new calculations.

At Bentley, it's only the highway mileage that changes for the 2007 Arnage with long wheelbase. It goes from 15 mpg to 14 mpg. City mileage remains at 10 mpg.

Likewise, the 2007 Dodge Ram full-size pickup truck with four-wheel drive and HEMI V8 keeps its 18-mpg highway rating in the new calculations. Its city mileage drops just 1 mpg, to 13 mpg.

Even small cars are seeing declines in their fuel economy ratings.

The 2007 MINI Cooper with manual transmission has ranked as the top mini-compact in fuel economy with 40 mpg on the highway and 32 mpg in city driving.

In 2008, these numbers drop 10 percent, to 36 mpg on the highway, and 15.6 percent, to 27 mpg in city driving.

Why the Change?
For years, government officials have heard complaints from consumers that they usually don't get the mileage that's posted on the window stickers of new cars. The issue didn't attract a lot of attention until this decade when fuel prices rose over and over again and many more drivers started watching their fuel use.

Then the Bluewater Network environmental group in San Francisco formalized the complaints into a petition in June 2002. The government has been studying the matter and drawing up the changes ever since.

How Vehicle Testing Is Changed
Since the EPA began gathering mileage ratings in the 1970s, it has not provided figures derived from real-world driving.

Instead, the numbers stem from laboratory testing required by law to measure vehicle emissions. Basically, government officials measure the amount of carbon expelled in a vehicle's exhaust during testing to deduct how much fuel has been consumed. (This system came about after auto companies complained about the cost and burden of having to do an additional test just for fuel economy.)

During testing, new vehicles—usually pre-production models—are "driven" on a dynamometer so wheels rotate as they would on a road. But there's no wind or other weather conditions in the laboratory.

There also are no changes in road grade, and the test driver must follow a strict "schedule" of accelerator and speed situations that are meant to mimic real-world driving.

The results are then put into a complicated formula to derive the mileage artificially. This formula was adjusted once before—in the 1980s—to decrease mileage figures and better reflect real driving conditions.

This time, the EPA is adding new testing to account for drivers accelerating more quickly than in previous tests, driving at higher speeds and using air conditioning. It also is taking into account lower outside temperatures. All these factors can lower a vehicle's fuel economy.

Note, however, that even the new EPA figures are estimates and do not necessarily indicate the exact mileage every driver will get.

Mileage is affected by driver habits, weather, the use of power-draining accessories, terrain, the condition of a vehicle, even tire pressure.

Some Worry in the Industry
Auto officials had years to give the EPA input into the new testing, and many worry that the arrival of the new, lower numbers coincides with increased global warming concerns and a Democratic majority in Congress.

The result could be a stronger push for more fuel-saving autos—both in showrooms and in Washington.

Already, the Bluewater Network has petitioned the government to help create a market for electric plug-in vehicles.

And some lawmakers are discussing whether to demand higher fuel economy from automakers in their annual Corporate Average Fuel Economy numbers, which are unaffected by the EPA adjustments.

Ann Job is a freelance automotive writer.

In the market for a new car? MSN Autos is pleased to provide you with information and services designed to save you time, money and hassle. Click to research prices and specifications on any new car on the market or click to get a free price quote through MSN Autos' New-Car Buying Service.